Republican presidential nominee Donald Trump, behind in the polls, has started claiming that the election will be stolen from him because it is “rigged.” When Trump talks about the election being stolen from him, he seems to be referring to a range of issues, including voter fraud and the media allying against him. He also said this week that he expects more than a million “deceased individuals” to vote against him.

These claims have the potential to resonate with many Americans who already question the integrity of this country’s elections. A September Washington Post-ABC News poll found that 46 percent of registered voters believe that electoral fraud happens at least somewhat often.

But stealing an election in this country isn’t easy. In fact, experts say it’s nearly impossible, given how voting works. And documented instances of voter fraud are very rare. Wendy R. Weiser, director of the Democracy Program at the Brennan Center for Justice, said the rate of fraud is smaller than the rate of Americans being struck by lightning.

Below is an explanation for why rigging an election is so unlikely.

What would rigging an election entail?

Rigging an election would demand a widespread, nationwide effort with the two major parties colluding at every level. This is why election law experts say it would be extremely difficult, if not impossible, to “rig” an election. In this country, voting is an open, multi-step process with scores of witnesses from both parties each step of the way.

Chris Ashby, campaign finance and election lawyer at Virginia-based Ashby Law, points out that American elections are held in open, public rooms, such as school gyms and community centers.

“There are no back rooms, secret doors or hidden hallways,” Ashby wrote recently. The ballots, voting machines and election materials are locked and sealed when they arrive in the voting place, and when they are removed after the election is over, they are locked and sealed again.

In most states, there are “poll observers” in each county who have been chosen and trained by both the Republican and Democratic parties to watch for problems or efforts to disenfranchise voters during the voting process. The poll observers are allowed to watch the poll workers and other election officials, who have also undergone training to run the polling places and help conduct the election.

Voters use equipment that is publicly tested and observed by party representatives and representatives of the campaigns, Ashby said. After it’s tested, voting equipment is locked and sealed.

Stealing an election requires the cooperation of the Republicans and Democrats who are the polling place election officials, along with the poll watchers from each party who are watching the election officials conduct the election, Ashby said. It would also need, he points out, the cooperation of the another group of Republicans and Democrats after the election who are watching the counting of ballots.

Sen. Marco Rubio (R-Fla.), who is running for reelection, said during a debate this week that Trump should stop saying the election is being rigged. “We have 67 counties in this state, each of which conduct their own elections. I promise you there is not a 67-county conspiracy to rig this election,” he said, pointing out that Republicans control many of the positions that oversee ballots and monitor results. “There is no evidence behind any of this. . . . He should stop saying that.”

The FBI released a statement Tuesday saying that agents work with federal, state and local officials to ensure a fair election process. “The FBI takes allegations of election-related violations of federal law seriously and encourages citizens to call their local field office to report an election crime,” the statement said.

How do I know that the votes are being counted fairly?

Every jurisdiction has multiple overlapping systems in place to ensure fair vote counts, according to the Brennan Center for Justice at the New York University School of Law.

After voting is over in each polling place, representatives for the political parties and the candidates literally watch the election officials count the votes. They also later participate in what is called a public canvass where the election results are gone over again to make sure they are right.

“The outcome of the election is not official until the canvass is finished, sometimes several weeks after Election Day. The purpose of the canvass is to account for every ballot cast and ensure that every valid vote cast is included in the election totals,” according to the bipartisan federal Election Assistance Commission. “This involves accounting for every absentee ballot, every early voting ballot, every ballot cast on Election Day, every provisional ballot, every changed ballot and every overseas and military ballot.”

Richard L. Hasen, an election-law expert at the University of California at Irvine and the author of “The Voting Wars: From Florida 2000 to the Next Election Meltdown,” said that counting votes at county offices and other places is “a transparent act with Republicans, Democrats and good government groups watching the counting.”

“When voting anomalies occur, generally because of human error, they are quickly caught and publicized on Twitter, and then corrected,” Hasen wrote this week in an article in Slate. “Most election administrators doing the tabulating and reporting are dedicated public servants who want the process to be as transparent as possible to promote public confidence.”

Are there a lot of dead people voting?

No. Weiser says she often hears the claim that there are hordes of dead people voting but that this has been debunked repeatedly.

“There have been a handful of cases where votes have been cast in the name of dead people, and those have typically been minuscule in scale (like someone voting in the name of his or her recently deceased spouse) or involved ballot-box stuffing by unscrupulous election insiders,” Weiser said in an email.

She added: “There has been no incident in over a century in which people were able to impact an election by mobilizing fraudsters to impersonate dead people at the polls.”

Deceased voters, just like people who move, do linger on the voter rolls, as the Pew Center on the States has documented, Weiser said. But to address that problem, she said that voter registration systems need to be modernized.

How hard is it to vote more than once in a presidential election?

Extremely hard, according to election experts. Hasen said there is “zero evidence” this kind of fraud has occurred regularly and that you would to have vote multiple times on a massive scale to influence the election.

Hasen said that the only recent instance he knows involving multiple voting concerned a supporter of Republican Wisconsin Gov. Scott Walker and other Wisconsin Republicans.

“This man registered and voted about a dozen times in a few elections in multiple jurisdictions,” Hasen said. “He was caught and literally pleaded insanity, because such a plan is literally insane.”

Is there any evidence of widespread voter fraud with people impersonating others?

In one of the most comprehensive studies on voter fraud, Loyola Law School professor Justin Levitt, who is now the deputy assistant attorney general in the Justice Department’s Civil Rights Division, found 31 incidents of voter impersonation out of more than a billion ballots cast between 2000 and 2014.“Usually, only a tiny portion of the claimed illegality is substantiated — and most of the remainder is either nothing more than speculation or has been conclusively debunked,” Levitt wrote.

The Government Accountability Office concluded in a report to Congress that the Justice Department had found “no apparent cases of in-person voter impersonation . . . anywhere in the United States, from 2004 through July 3, 2014.”

Judge Richard Posner, a conservative judge on the U.S. Court of Appeals for the 7th Circuit, said “besides the risks to the politicians, think of how much it would cost to orchestrate an effective voter impersonation fraud, given the number of voters who must be bribed, and in amounts generous enough to overcome their fear of being detected.”